1.stumble into 闯入
Waiting for a gas cloud to stumble into their vicinity isn't a very efficient means of hunting.
等待气体云闯入其领地可不是有效率的捕猎方式。

2.fall off 跌落
According to the myth, the sailors believed after sailing for weeks without spotting land, they would fall off the face of the Earth.
根据传说，船员们认为如果一直航行数周而见不到陆地，他们将会从地球表面坠落。